1. Field of the Invention
This invention relates to data display arrangements of a type for displaying as an entity on the screen of a CRT (cathode ray tube) or other raster scan display device, a quantity of stored data which is accessed repeatedly for its display in a recurrent cycle of scanning lines, the displayed data being composed of discrete characters organized in character rows each comprising a plurality of character positions, and the shape of each discrete character being defined by selected dots of a dot matrix which constitutes a character format for the characters.
2. Description of the Prior Art
Data display arrangements of the above type are used in a variety of different applications. For instance, one such data display arrangement is used in conjunction with telephone data services which offer a telephone subscriber having a suitable video terminal (incoporating the arrangement), the facility of access over the public telephone network to a data source from which data can be selected and transmitted to the subscriber's premises for display. Examples of this usuage are the British and German videotext services Prestel and Bildschirmtext.
A data display arrangement of the above type includes, in addition to the CRT or other raster scan display device, acquisition means for acquiring from a data source transmission information representing data selected for display, a display memory for storing digital codes derived from the transmission information, and character generator means for producing, from the stored digital codes, character generating signals for driving the display device to produce the data display.
The character generator means normally includes a fixed character memory in which is stored character information identifying the available character shapes which can be displayed by the arrangement. This character information is addressed selectively in accordance with the stored digital codes in the display memory, and the information read out is used to produce the character generating signals for the data display. This selective addressing is effected synchronously with the scanning action of the display device, which scanning action may be effected with or without field interlacing.
With a view to extending the display facilities of a data display arrangement of the above type, it has been proposed to increase the number of character shapes which are available for selection to form a display by providing the arrangement with a number of so-called "dynamically redefinable character sets" (DRCS) which are available at a remote data source from which they can be transmitted selectively to the arrangement for temporary storage as part of the overall character memory. In principle, DRCS characters can have any alpha-mosaic (including alpha-numeric) shape, so that the total possible number of different shapes that can be made available is limited only by the character format used. Thus, DRCS characters can be defined as elements for constructing, with high resolution, graphics such as maps, and geometric and other drawings. (For the purposes of the present specification the term "DRCS character" is to be construed to mean a character the character information for which can be transmitted from a data source to the arrangement for temporary storage therein to form part of an overall character memory. The term "DRCS character set" is to be construed to mean a set of DRCS characters).
Hitherto, a data display arrangement of the above type, in which the display is "character-based" (i.e. the display is made up of discrete characters in specified positions), has generally been accepted as being less suitable for high resolution graphics display compared with a data display arrangement which provides a graphics display using traditional computer graphics techniques. With these techniques, received transmission information provides coded instructions for defining display elements such as vectors, arcs, circles, etc. These instructions are decoded as they are received into dot information for the display elements concerned, and this dot information is stored in the display memory for direct read-out to provide the display. This storage is in so-called "bit-map" form, each pixel (or dot) of the display being represented by at least one stored bit. The amount of storage used by the display memory for such a bit-map display is far greater than that required by the display memory for a character-based display for which whole character memory cells (e.g. a matrix of up to 120 pixels) at a time are identified in the display memory, rather than individual pixels.